In data processing systems, magnetic disc memory systems are frequently used because they have large storage capacity and require a relatively short time for a magnetic read/write head to access data contained anywhere on discs of the memory from the moment the head receives an instruction to access the data. The magnetic discs are driven at constant rotational velocity by an electric motor.
A magnetic disc carries coded data, usually in binary form, on both faces of the disc in concentric circular recording tracks having a width that does not exceed a few hundredths of a millimeter. The tracks are identified by allotting them an address or serial number j (j being a whole number) from 0 to (N-1), where N is the total number of recording tracks. The tracks having addresses (j-1) and (j+1) are adjacent tracks j.
Memories having a small storage capacity contain a limited number of discs (normally only one or two). In small memories, data are stored, i.e., recorded, on each of the disc faces by setting a large amount of space aside for the data intended to be processed by the data processing system of which the memories are a part. A small amount of space is set aside for data that enables the tracks to be located; these data indicate the track addresses and enable the magnetic head to be servo controlled to a position above the tracks. In the small area are also stored data which indicate whether or not the tracks contain faults.
For the sake of simplicity, a memory is considered which contains only a single disc. Preferably, each face of the disc is associated with a single magnetic read/write head, i.e., a magnetic read/write transducer. Current practice, as described in commonly assigned United States patent application Ser. No. 765,058, filed on Feb. 2, 1977, entitled "Method of Writing Addresses on a Magnetic Recording Medium", is for the data contained on each face of the disc to be distributed over equal adjacent circular sectors S.sub.0, S.sub.1 . . . S.sub.i . . . S.sub.n. Generally, one face of a disc is divided into several tens of sectors (usually 40 to 50).
When the magnetic disc face associated with the magnetic head passes in front of the head, sectors S.sub.0, S.sub.1, S.sub.2, etcetera, are read by the head in sequence. It is therefore said that sector S.sub.0 precedes sector S.sub.1, that sector S.sub.1 precedes sector S.sub.2, that sector S.sub.i precedes sector S.sub.i+1, and so on. In more general terms, if two items of information I.sub.k-1 and I.sub.k which follow one another along the same track j on the face are considered, item I.sub.k-1 precedes item I.sub.k if item I.sub.k-1 is read by the head before I.sub.k, or that item I.sub.k follows item I.sub.k-1.
The same reasoning is applied to groups of information items G.sub.k and G.sub.k-1 in a track (j+1) adjacent and abutting with track j.
Each sector S.sub.i is divided into two unequal areas. The larger area contains the data to be processed by the data processing system of which the disc memory is a part, while the smaller area contains data for locating the tracks and indicating faults. For the sake of simplicity, the data contained in the larger area is referred to as "data to be processed". In each sector, the smaller area is divided into a plurality of reference zones, one for each track so each track is associated with a single reference zone.
It is recalled that a bit is a binary 1 or 0 digit. The 1 or 0 may be expressed on a magnetic medium or as an analog or logic electrical signal. A logic signal is capable of assuming only two values called "logic or binary zero" and "logic or binary one"; an analog signal is a signal having a voltage that may vary continuously between two positive and/or negative extreme values. Any item of data or information recorded on the disc is referred to herein as a "bit".
To record a series of data items on a magnetic disc, a succession of small magnetic domains termed "elementary magnets" are formed on each track. These domains are distributed along the entire length of the track and have magnetic inductions with the same modulus and of successively opposing senses in a direction parallel to the surface of the disc. A data bit is represented by a change in the sense of magnetic induction, also termed a magnetization sense change. There are two different types of sense changes, namely:
when the magnetic head passes successive elementary magnets having negative and positive induction on the disc, the magnetization sense change is positive; and
when, on the other hand, the head passes successive elementary magnets having positive and negative induction, the magnetization sense change is negative.
The address of a track contains a number (p) of bits such that 2.sup.p is less than or equal to the number of tracks n. Each reference zone in sector S.sub.i, associated with a track having an address j, contains n cells (n being a whole number) C.sub.1, C.sub.2 . . . C.sub.k . . . C.sub.n. The cells are preferably arranged so each of a number (p) of cells contains two bits; one bit represents a portion of position control information, while the second bit represents a portion of the address for the track of serial number j. Another type of cell contains two fault-indicating bits which indicate whether a portion of track j within sector S.sub.i+1 (following sector S.sub.i) does or does not contain faults. The cells are described in further detail in the commonly assigned United States patent application entitled "Method of Writing Information Relating to Faults in a Magnetic Recording Medium" filed as Ser. No. 835,402 on Sept. 21, 1977.
The two magnetization sense changes which correspond to the two bits of each cell are of the same kind. Each change can occupy one of only two predetermined positions in the cell. The value of the bit represented by the change depends upon the position which the change occupies, as described in the above mentioned patent application Ser. No. 765,058. Thus, if a cell in a reference zone containing track locating data is considered, the position-control bit corresponds to the first change while the track address bit for the zone is the second change. If the magnetization sense change corresponding to the address bit occupies a first position (the position first encountered by the magnetic read head when the face of the magnetic disc which is associated with the head passes in front of it) the bit is equal to 0. If the change in magnetization occupies the other of the two predetermined positions, termed the "second position", the bit is equal to 1. The same rules apply both to the position-control bits and to the fault indicating data.
When the magnetic head encounters a series of magnetization sense changes representing a reference zone, it emits a series of analog signals which are shaped into a series of logic pulses by shaping circuits. The beginning of a reference zone is indicated by a special pulse.
To convert a magnetic transition in a reference zone to an analog pulse and thence into a binary logic pulse, it is necessary for the analog pulse amplitude to be above a certain threshold level. In current practice, the threshold is relatively low and is typically about 25% of the maximum amplitude of an analog signal derived by a read head. The value of a track identifying bit in the reference zone is determined in the manner described in French Patent application No. 76.28169, entitled "Method of Reading Addresses on a Magnetic Recording Medium and Arrangement for Putting it into Practice", commonly assigned with the present application. In particular, track identifying bits in reference zones are determined in accordance with the prior art by the time position of a logic pulse corresponding to a magnetic polarity translation relative to a logic pulse indicating the beginning of the reference zone. Hence, in the prior art the magnetic transition P.sub.k corresponding to a binary bit I.sub.k in a reference zone has a logic pulse associated with it. The logic pulse occupies a clearly defined time position P.sub.k with regard to a special pulse indicating the beginning of the reference zone.
A spurious signal from a reference zone is defined in the present specification and claims as any signal which is derived by a magnetic head when the reference zone is passing in front of the head, but which does not correspond to a reference zone magnetic polarity transition. Hence, a spurious signal is any signal which does not occupy one of the positions P.sub.k. Spurious signals occur for various reasons, such as a variation in the position of a magnetic reading head relative to a disc face associated with the head. Also, spurious signals may occur because the magnetic transitions are not properly recorded on the disc face, or because of the presence of dust between the disc face and magnetic reading head.
There are certain disadvantages concerning the way in which data have been written into reference zones of magnetic discs in the past. In particular, the relative linear speed of tracks on the disc and the magnetic read head varies, as a function of track radial position, whereby the track versus head linear speed decreases as the disc center is approached. In consequence, there are uncertainties in determining the positions P.sub.k of pulses corresponding to transitions T.sub.k in a reference zone. There are also variations in the duration and amplitude of the logic or analog pulses derived in response to output signals of the magnetic reading head. These variations result in a relatively high probability that read/write circuits of the disc memory will respond to a spurious signal and thereby provide an erroneous output.
In the prior art, signals derived from a magnetic head associated with a magnetic disc face and which represent track identifying information from a reference zone are supplied to an apparatus for positioning the head above the disc. The head positioning apparatus enables the head to be moved radially from a first track A, above which the head is initially situated, to a second track B, from which it is desired to read information. The track positioning apparatus also enables the magnetic head to be held exactly centralized above track B for the time required to read information from it. To enable information from track B to be read from the head as quickly as possible and with maximum accuracy, the time required for the head to move from track A to track B should be as short as possible. It is necessary, however, not to read information until the head arrives at track B, at as centralized portion relative to the track as possible. To meet these conditions, none of the signals read from the reference zone should be of a spurious nature and they must correspond to transitions in the zone.